I would definitely not go with anything frilly next to Emerson, so that automatically eliminates Ava, Penelope, Ella, and Lillian. I would also eliminate Sydney, Phoebe, and Finley because they end with the "ee" sound like your last name does. So that leaves Quinn and Hayden. Hayden is a great big no for me because it rhymes with way too many popular names. She would be lost in a sea of Aidens, Kaydens, Bradens, Jaydens, and Zaidens and all their spelling variations. For me, Quinn is the sure winner here. Emerson and Quinn sound great as sisters. Congrats!